The most commonly employed photographic elements are those which contain a radiation-sensitive silver halide emulsion layer coated on a support. Although other ingredients can be present, the essential components of the emulsion layer are radiation-sensitive silver halide microcrystals, commonly referred to as grains, which form the discrete phase of the photographic emulsion, and a vehicle, which forms the continuous phase of the photographic emulsion.
It is important to recognize that the vehicle encompasses both the peptizer and the binder employed in the preparation of the emulsion layer. The peptizer is present during the precipitation of the grains to avoid their coalescence or flocculation. While a variety of hydrophilic colloids are known to be useful peptizers, it has been well known that the preferred peptizers are forms of gelatin, e.g., alkali-treated gelatin (cattle bone or hide gelatin) or acid-treated gelatin (pigskin gelatin), and gelatin derivatives, such as acetylated or phthalated gelatin.
There are some problems, however, associated with nitrogen- and sulfur-containing vehicles such as gelatin. Gelatin absorbs ultraviolet light, particularly in the regions of the spectrum less than 300 nm. Although silver halide is light sensitive throughout the region 200 to 400nm, its sensitivity throughout much of this region is masked by the absorption of light by the vehicle in which it is dispersed as well as by glass usually used for lenses in exposure devices. Special techniques have been required to minimize the concentration of gelatin in the light-sensitive layers of spectroscopic plates designed for recording ultraviolet light. In addition, some metal ions, potentially of value as dopants, are so tightly adsorbed to vehicles such as gelatin that they are not incorporated into the precipitated grains.
Materials useful as peptizers, particularly gelatin and gelatin derivatives, are also commonly employed as binders in preparing an emulsion for coating. However, many materials are useful as vehicles, including materials referred to as vehicle extenders, such as latices and other hydrophobic materials, which are ineffective peptizers. A listing of known vehicles is provided by Research Disclosure, Vol. 308, Dec. 1989, Item 308119, Section IX, Vehicles and vehicle extenders. Research Disclosure is published by Kenneth Mason Publications Ltd., Emsworth, Hampshire, P010 7DD, England. Among the vehicles listed are some polysaccharides: dextran, gum arabic, pectin, and agar-agar. Only citrus pectin has been described by Fallesen U.S. Pat. No. 2,343,650 as a peptizer for silver halide but makes no reference to its absorption of ultraviolet light.